Electronic image sensors are typically comprised of a large number of very small light detectors, together called “pixel arrays”. These sensors typically generate electronic signals that have amplitudes that are proportional to the intensity of the light received by each of the detectors in the array. Electronic cameras comprise imaging components to produce an optical image of a scene onto the pixel array. The electronic image sensors convert the optical image into a set of electronic signals. These electronic cameras typically include components for conditioning and processing the electronic signals to allow images to be converted into a digital format so that the images can be processed by a digital processor and/or transmitted digitally. Various types of semiconductor devices can be used for acquiring the image. These include charge couple devices (CCDs), photodiode arrays and charge injection devices. The most popular electronic image sensors utilize arrays of CCD detectors for converting light into electrical signals. These detectors have been available for many years and the CCD technology is mature and well developed. One big drawback with CCD's is that the technique for producing CCD's is incompatible with other integrated circuit technology such as MOS and CMOS technology, so that processing circuits and the CCD arrays must be produced on chips separate from the CCD's. Another drawback is that the CCD sensors consume large amounts of energy (as compared to cameras with CMOS sensors) and require high rail-to-rail voltage swings to operate CCD. This can pose problems for today's mobile appliances, such as Cellular Phone and Personal Digital Assistant.
Another currently available type of image sensors is based on metal oxide semiconductor (MOS) technology or complementary metal oxide semi-conductor (CMOS) technology. These sensors are commonly referred to as CMOS sensors. CMOS sensors have multiple transistors within each pixel. The most common CMOS sensors have photo-sensing circuitry and active circuitry designed in each pixel cell. They are called active pixel sensors. The active circuitry consists of multiple transistors that are inter-connected by metal lines; as a result, this area is opaque to visible light and cannot be used for photo-sensing. Thus, each pixel cell typically comprises a photosensitive region and a non-photosensitive region. In efforts to minimize this problem and to increase the exposure in the photo-sensitive region of each pixel, it is a known practice to utilize micro-lens arrays with a micro-lens positioned over each pixel directing light, which would otherwise illuminate an opaque region, onto the photosensitive region of the pixel. In these conventional CMOS design, the photodiode area is typically only about 30% of the total pixel area and the rest of the 70% are used for pixel circuit. This 30% is called fill-factor. Practitioners in the industry have found that a hemispherical shape structure made of transparent polymer can gear the light into the photodiode area and in effect increase the fill-factor by two to about 60%.
Attempts have been made to produce small visible light cameras using CMOS sensors on the same chip with processing circuits. One such attempt is described in recently issued U.S. Pat. No. 6,486,503. Small cameras using CMOS sensors typically use less energy than CCD sensors and may provide a solution for energy consumption; but the traditional CMOS-based small cameras suffer low light sensing performance, which is intrinsic to the nature of CMOS active pixel sensors caused by shallow junction depth in the silicon substrate and its active transistor circuitry taking away the real estate preciously needed for photo-sensing.
U.S. Pat. Nos. 5,528,043 5,886,353, 5,998,794 and 6,163,030 are examples of prior art patents utilizing CMOS circuits for imaging which have been licensed to Applicants' employer. U.S. Pat. No. 5,528,043 describes an X-ray detector utilizing a CMOS sensor array with readout circuits on a single chip. In that example image processing is handled by a separate processor (see FIG. 4 which is FIG. 1 in the '353 patent. U.S. Pat. No. 5,886,353 describes a generic pixel architecture using a multi-layer hydrogenated amorphous silicon layer structure, either p-i-n or p-n or other derivatives deposited on top of an array of CMOS pixel circuits. This architecture is referred to a “photoconductor on active pixels” (POAP). U.S. Pat. Nos. 5,998,794 and 6,163,030 describe various ways of making electrical contact to the underlying CMOS circuits in a pixel in a POAP architecture. All of the above US patents are incorporated herein by reference.
An important advantage of the POAP structure is that the photodiode portion of each pixel in the pixel array, which may be many thousands or millions of pixels, can be laid down on top of the pixels in a continuous process as a non-segmented continuous multilayer photodiode layer. This greatly simplifies sensor fabrication. However, this architecture also permits in some cases charges produced by photons in the region above one pixel to drift or be attracted to a collection electrode of a neighboring pixel. This is more likely to happen toward the end of a charge integration period when the potential across a highly illuminated pixel is reduced compared to a much more lightly illuminated neighboring pixel where the potential would have remained close to a reset potential. This problem is referred to in the sensor industry as “crosstalk”. One of the biggest challenges in using POAP technology is to avoid pixel-to-pixel cross talk without pixel patterning of the photo-sensing layers. This challenge becomes more difficult as the pixel size decreases, especially when the sizes decrease below about 3 microns.
A need exists for improved camera technology which can provide electronic cameras with pixel sizes in the range of 3 microns and smaller with minimum pixel-to-pixel crosstalk.